The present invention relates to a cassette transfer mechanism for transferring a cassette storing substrates such as semiconductor wafers to a substrate unloading position at which the substrate is taken out from the cassette.
When a semiconductor wafer is treated in various processes including a film formation process, an etching process, and a thermal oxidation process, a cassette storing a plurality of semiconductor wafers (e.g. 25 wafers) is first placed in a cassette chamber. Thereafter, one of the wafers stored in the cassette is transferred from the cassette chamber by a transfer arm through a transfer chamber to a process chamber under a vacuum.
When the cassette is set within the cassette chamber, the cassette is first set on a stage of the cassette chamber. At this point, the wafer loading entrance of the cassette is turned upward. Thereafter, the cassette is rotated about 90 degrees and simultaneously taken into the cassette chamber by a drawbridge-form moving mechanism installed therein. Consequently, the upwardly-provided wafer loading entrance turns sideward in the horizontal direction.
Although the transfer chamber has only one transfer arm therein, a plurality of cassette chambers, for example, two cassette chambers, are usually connected to the transfer chamber. For this construction, one transfer arm must access two cassettes in the cassette chambers. Hence, each cassette must be turned in such a way that the wafer loading entrance facing in the horizontal direction corresponds to the accessing direction of the transfer arm.
FIG. 9 shows a cluster tool having process chambers for processing a semiconductor wafer W. The wafer W is sent to a process chamber by the aforementioned serial operation and processed therein. The cluster tool mainly consists of two process chambers (2, 4), a transfer chamber 6 connected to process chambers (2, 4), and two cassette chambers (8, 10) connected to the transfer chamber 6. Individual chambers are communicable with each other via a gate valve G1 closed airtight. The transfer chamber 6 has a flexibly-bending rotatable arm, for example, a multi-joint transfer arm 12, installed therein, as shown in FIG. 10. The transfer arm 12 plays a part in loading/unloading the semiconductor wafer W into/from the cassette C housed in each of cassette chambers (8, 10).
When the cassette C is mounted on a stage placed outside a gate door (G1) of each of the cassette chambers (8, 10), it is often placed in the X-axis direction with a wafer loading entrance 14 turned upward (in the Z-axis direction), depending upon the entire construction of the system or other conditions. To take the cassette C into the cassette chambers (8, 10) and to take out a semiconductor wafer W from two cassettes (C, C) of the cassette chambers (8, 10) by one transfer arm 12, the wafer loading entrance 14 of the cassette C must be brought to face the center of the transfer arm 12 (transfer arm access direction A) by raising the cassette C in the vertical direction and rotating it horizontally.
The aforementioned operation is actually performed in apparatuses disclosed in, for example, U.S. Pat. Nos. 5,186,594 and 5,507,614. In U.S. Pat. No. 5,186,594, a cassette C is first mounted on a stage outside a cassette chamber 8 with a wafer loading entrance 14 turned upward. Then, the cassette C is rotated around the Y-axis by a drawbridge-form moving mechanism by 90 degrees. In this manner, the cassette C is taken into the cassette chamber 8; at the same time, the wafer loading entrance 14 facing upward (Z direction) is turned sideward in the horizontal direction (X-direction). Thereafter, the cassette C is rotated around the Z-axis by a predetermined angle .theta. by a pivot mechanism. In this rotation, the wafer loading entrance 14 is turned to the center of the arm 12 (see FIG. 11). In U.S. Pat. No. 5,507,614, a cassette C can be brought into a desired position in one step by rotating an inclined axis having a predetermined angle with the gravity direction. Consequently, the wafer loading entrance 14 is turned toward the center of the arm 12.
In the apparatus disclosed in U.S. Pat. No. 5,186,594, the cassette chamber 8, 10 (vacuum chamber) contains two moving mechanisms, namely, a drawbridge-form moving mechanism (for rotating the cassette C around the Y axis by 90 degrees) and a pivot mechanism (for rotating the cassette C around the Z-axis by a predetermined angle). Construction of the cassette chamber is complicated by a plurality of moving mechanisms. Also, due to a plurality of moving mechanisms, the manufacturing cost and the amount of generated particles increase.
On the other hand, the apparatus disclosed in U.S. Pat. No. 5,507,614 employs the inclined rotation axis. When the inclined rotation axis is rotated, the amount of particles generated due to the rotation increases because of the uneven movement of the rotation axis, as compared to the horizontal rotation. The diagonal rotation of the cassette C requires a broader space, increasing the size of the chamber.